[LLVMdev] Decouple LoopVectorizer from O3

[Anadi Mishra]

Hi David,

Thanks for the suggestion, here is the link for the bug report.

http://llvm.org/bugs/show_bug.cgi?id=15720


Best,
Anadi.


On Mon, Apr 15, 2013 at 12:30 PM, David Blaikie <dblaikie at gmail.com> wrote:
> Just an FYI: it's often handy to mention the PR number when a thread is
> concluded by filing a bug. That way other people reading (now, or more
> importantly, later) can follow the issue through to the bug and its
> resolution
>
> On Apr 11, 2013 4:24 PM, "Anadi Mishra" <reachanadi at gmail.com> wrote:
>>
>> Done.
>>
>> Best,
>> Anadi.
>>
>>
>> On Thu, Apr 11, 2013 at 7:01 AM, Nadav Rotem <nrotem at apple.com> wrote:
>> > Hi Anadi,
>> >
>> > Yes, this is a bug in the loop vectorizer. The loop vectorizer expects
>> > only
>> > one loop counter (integer with step=1).  There is no reason why we
>> > should
>> > not handle the case below, and it should be easy to fix.  Interestingly
>> > enough if you reverse the order of iterations and count from SIZE to
>> > zero,
>> > the loop vectorizer would vectorize it.  If you open a bugzilla report
>> > and
>> > assign it to me I will look at it.
>> >
>> > Example:
>> >
>> > int foo(int *aa, int *bb, int SIZE) {
>> >   int count = 190;
>> >   for(int i=SIZE;i > 0;i--) {
>> >     aa[i] = bb[i] + count;
>> >     count++;
>> >   }
>> > }
>> >
>> >
>> > Thanks,
>> > Nadav
>> >
>> >   The loop vectorizer has some LSR-like capabilities for merging
>> > multiple
>> > induction variables into a
>> >
>> >
>> > On Apr 10, 2013, at 7:52 PM, Anadi Mishra <reachanadi at gmail.com> wrote:
>> >
>> > Hi Nadav,
>> >
>> > I tried your suggestion by changing the condition to :
>> >
>> > 189   if (LoopVectorize && OptLevel >= 0)
>> > 190     MPM.add(createLoopVectorizePass());
>> >
>> > and compiled. Then I used the following command:
>> >
>> > opt -mtriple=x86_64-linux-gnu -vectorize-loops
>> > -vectorizer-min-trip-count=6 -debug-only=loop-vectorize -O1-S -o
>> > example1_vect.s example1.s
>> >
>> > where example1.s is IR generated by
>> >
>> > clang -S -emit-llvm example1.c
>> >
>> > example1.c contains the following loop:
>> >
>> > for(int i=0;i<SIZE;i++)
>> >        {
>> >                aa[i] = bb[i] + count;
>> >                count++;
>> >                printf("a[%d] is %d", i,aa[i]);
>> >        }
>> >
>> > LV runs and says "LV: Found too many inductions".
>> >
>> > I am guessing that this because some of the loop passes are missing.
>> > Could you please shed some light on what passes should be used to
>> > prepare the IR for LV?
>> >
>> >
>> > Best,
>> > Anadi
>> >
>> >
>> > On Thu, Apr 11, 2013 at 2:48 AM, Nadav Rotem <nrotem at apple.com> wrote:
>> >
>> >
>> > Hi Anadi,
>> >
>> > In the file PassManagerBuilder.cpp you can change the lines below to get
>> > rid
>> > of the O3 restriction.
>> >
>> > 189   if (LoopVectorize && OptLevel > 2)
>> > 190     MPM.add(createLoopVectorizePass());
>> >
>> > Nadav
>> >
>> >
>> >
>> >
>> > On Apr 10, 2013, at 5:39 PM, Anadi Mishra <reachanadi at gmail.com> wrote:
>> >
>> > Hello,
>> >
>> > I am trying out the LoopVectorizer(LV) pass and would like to decouple
>> > it
>> > from O3 which is currently required to run LV. I want to do this because
>> > I
>> > want to understand the behaviour of LV by trying simple loops but the O3
>> > mostly optimises away the loop body.
>> >
>> > Any ideas would be appreciated.
>> >
>> > Best,
>> > Anadi.
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>> >
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev